Positions of H-free WR stars observed in the MW and evolutionary tracks of He stars with different masses. The initial masses from bottom to top are 4, 6, 8, 12, 16, 20, 25, 30, and 35 M_⊙. The input parameters are given in the top left corner of every panel. The green and blue lines represent the WN and WC sequences, respectively. The gray lines represents the evolutionary tracks of the He stars without the EIS obtained using MLT++. The red dotted lines represent the HeZAMS without the EIS, while the red solid lines represent the HeZAMS with the EIS. The orange lines correspond to the ZAMS of normal stars. The observational data of the WN, WC, and WO stars in the MW were taken from Hamann et al. (2019) and Sander et al. (2019).